Fears for the accountability of Egypt's police force have deepened after the only police officer jailed for his part in the deaths of protesters since the overthrow of Mohamed Morsi last summer had his conviction quashed on appeal.

Lt Col Amr Farouq was originally given a 10-year jail term for his role in the gassing to death of 37 prisoners last August. A Guardian investigation, citing testimony by police witnesses and survivors, alleged that the victims had died after teargas was fired through the windows of a crammed prison truck for which Farouq was responsible.

But this weekend, Farouq had his conviction reversed on appeal, and the case was sent to retrial.

Rights activists said the move raised questions about the accountability of Egypt's police force, who are frequently accused of excessive force and malpractice, but rarely brought to trial.

Mohamed Elmessiry, Egypt researcher for Amnesty International, said: "Egypt's judiciary is fast to try and sentence Morsi supporters or any dissenting voices, but has not yet found guilty any security officers involved in the death of hundreds of protesters on 14 August and the extra-judicial killing of these 37 prisoners. The judiciary must bring those responsible to justice, including at senior levels."

The father of one of Farouq's alleged victims, Dr Gamal Siam, an economist, nevertheless saw positives in a retrial that he hoped would allow for more of the incident's perpetrators to be charged, and for prosecutors to seek tougher sentences.

"I am sending the case back to the prosecutor," said Siam, whose son Sherif died in the truck. "There are a lot more people who should be in the case, but who were not. And I hope that in the retrial they will be accused of murder rather than for manslaughter."

Last August, outside Abu Zaabal, 37 prisoners trapped in the back of a van were allegedly gassed to death having been held for six hours in temperatures close to 40C. Patrick Kingsley talks to the survivors